1. Field of the Invention
The present invention relates to the field of fluid flow. More specifically, the present invention relates to the flow of viscous fluids in a fluid transportation system. An example is the flow of heavy crude oil in tubular transport bodies.
2. Background of the Invention
The production of heavy oil reserves is becoming increasingly useful to the petroleum industry. This is because the increasing value of oil reserves makes the production of heavier hydrocarbons more cost-effective. The cost of producing and then transporting heavy oil is greater than the costs for lighter oils because of its high viscosity. High fluid viscosity leads to increased friction within the fluid transportation system due to the shear stresses acting between the surfaces or walls of tubular members and the fluid being transported. This causes pressure drops in the fluid transportation system. In extreme situations, the viscous fluid being transported can stick to the walls of the tubular members, particularly at points of sharp flow direction change.
Most heavy oil and bitumen is transported by providing an additive to reduce the oil viscosity. The most common methods for reducing viscosity involve either blending the heavy oil with a low viscosity hydrocarbon diluent, or upgrading the heavy oil through early conversion and/or separation.
A known concept for reducing pressure drops for fluid transportation systems carrying heavy oil is to use core annular flow. The method involves forming a biphasic flow system wherein a higher viscosity fluid is the “core,” and a lower viscosity fluid is injected as a surrounding “annulus.” The biphasic fluid is introduced into the fluid transportation system, such as a pipeline, and propagated through the length of the fluid transportation system. For heavy oil transport, the heavy oil is the core and water is the annulus.
Core annular flow of heavy oil has been tested; however, such core annular flows in fluid transportation systems have not been widely practiced. One obstacle is that conventional tubing and pipeline conduits have an affinity for adhesion of heavy oil. Several patents describe the reduction of friction within the pipeline flow regime. For instance, U.S. Pat. No. 3,520,313 discloses the use of so-called polymeric drag reducers. These polymeric drag reducers include polyacryl amides, polyalkylene oxides, polyvinyl acetates, and polyvinyl sulfonic acids. U.S. Pat. No. 3,977,469 discloses the placement of an oleophobic film-forming agent in the water phase. This oleophobic film-forming agent is an aqueous solution of a water-soluble salt selected from silicates, borates, carbonates, sulfates, phosphates and mixtures thereof. Further, U.S. Pat. No. 5,385,175 discloses the use of a conduit wherein the inner surface is substantially hydrophobic and oleophobic.
Also, various patents describe the use of hardware and flow systems for moving biphasic fluid. Examples include U.S. Pat. Nos. 3,502,103; 3,826,279; 3,886,972; and 3,977,469. These tools and systems are utilized to reduce contact between the oil and pipe walls, resulting in lower pressure drops and higher, more stable flow rates.
Fluid stabilizers have been added to core flow systems in an attempt to facilitate the movement of oil through the annular water regime. For instance, G.B. Pat. No. 159,533 describes the use of stabilizers, such as silicates (Na2SiO3), phosphates, borates, and sulfates. U.S. Pat. No. 5,988,198 (the '198 patent) describes the use of colloidal silica and clay as part of a self-lubricating flow system. The '198 patent particularly provides a process for transporting de-aerated bitumen froth containing 20 to 40% by volume froth water. The froth water contains colloidal-size particles with amphilic properties, which are particles that are hydrophilic but readily stick to the crude oil. The particles are carried through the pipeline to establish self-lubricated core-annular flow of the de-aerated bitumen froth. U.S. Pat. No. 3,892,252 discloses a method for increasing the flow capacity of a pipeline used to transport fluids by introducing a micellar system into the fluid flow. The micellar system comprises a surfactant, water and a hydrocarbon, and may be carried through fluids or a pig.
The economics of core annular flow have been further hindered by the large quantities of water utilized and the difficulties in maintaining the core annular flow regime under shear. In this respect, the shear forces acting at the water-heavy oil interface induce destabilization of the fluids. Therefore, a need exists for an improved core annular flow system. Further, a need exists for a core annular flow system wherein the shear-induced destabilization of the oil-water interface is reduced.